Appreciate this is probably due to the cold but my jets aren't squirting. Reservoir is still liquid. Is it worth emptying / syphoning off what's in the bottle then pouring some hot water in? Not even 100% its frozen pipes because it got to a tropical 3 degrees yesterday and they still didnt work...

Try using fluid with a lower freezing point. The 3 for 2 ready mix from a well known shop is only down to -6 and what happens is it exits the jets, some residue fluid hangs about the jets and is hit by the cold air and freezes, preventing more fluid coming out. You can make up your own fluid using ammonia/water/white vinegar solution as Ammonia freezes at -79 so even diluting down to 1 part to 4 parts water will keep it at -20. When buying fliud from shops, look for the ones with 'highly flamable' warnings as they will contain higher concentrates of alcohol. Think Vodka in the freezer... it doesn't freeze in your freezer, so high alcohol content in washer fluid helps reduce freezing.